Seeds are a non-solid block, and they are used to plant Crops on plowed soil / tilled land (see Plow). They cannot be consumed directly.
Seeds can be obtained by processing their Crops in the Processor. Currently every type of Crops can be processed into 2 according Deeds each. Additionally, Wheat Seeds can be randomly obtained when collecting Tallgrass or Savannah Tallgrass (even after you've placed it yourself).
Most other Crops can be found by randomly scouring the lands, such as Lettuce and Turnips in most Forests, Turnips also often in Swamplands, on Grass or Dead Grass, but Horned Melons nearly always only on Jungle floors, meaning Detritus.
Very rarely additional Seeds will be obtained while harvesting Crops. If you harvest your Crops too soon (like when they're still Seedlings), you will only get a Seed each, but no Crops.
You should plant a Seed on each block of tilled land quickly (which also applies to the time right after the harvest of Crops), otherwise the tilled land will turn into Dirt.
Please note that Seeds will stay fallow as long as they have not been moistened (by placing liquid adjacently to the tilled land with the Seed on top). "Fallow" will be visible in red letters below the name of the Seed when you point your cursor at the Seed. Fallow Seeds will not start to sparkle, growing Seeds usually do.
However, even moistened Seeds will also stay fallow in unsuitable Biomes or circumstances, like in too cold (snowy, icy) areas, too high up (above altitudes of ca. 150, which you can find out about by typing /sim into your chat) or too hot (especially on the Lava layer or in areas where heat meters show up), as well as in or close to Swamplands. Sometimes, Seeds might even stay fallow in Forests or other usually suitable Biomes for unknown reasons.
All Crops will need Water, Bog Water or Mineral Water 1 block adjacent to their block to grow. You cannot use Tar, Molasses or Corrupted Water to moisten Crops. The liquid can be placed directly below the block of tilled land, or can be placed diagonally. It is sufficient to only have one corner of the tilled land touch the block where you'll place the liquid to moisten the acre.
See Farming for more details about this.
The type of liquid used will not speed up the growth of Seeds into Crops, and will not influence the growth of Crops in any other way. If you do not place liquids adjacent to the block of tilled land, the Seed you will place there will simply stay fallow, but it can still start to grow if you place liquid next to the tilled land a bit later on. If the Seed should stay fallow, you should pick it up and place it again, then it should start to sparkle immediately and start to grow into a Crop.
In Creativerse, Crops do not need any sunlight nor artificial light to grow. You can plant Seeds for Crops underground or into completely dark rooms, as long as a suitable type of liquid is adjacent to the tilled land where you'll place the Seed. It doesn't matter if you plant Seeds at day or night either.
Crops will grow on game worlds no matter if you're online or offline, close by or far away. Seeds will grow into Seedlings within ca. 11 RL minutes. Then they will grow into Sprouts, and at last into Crops. The Crops will stay ripe for 24 hours, which is the best time to harvest them. Afterwards they will turn into "grown" Crops, and the harvest will be a little less than when harvesting "ripe" Crops. Grown Crops will not ever wither.
Crops do not require fertilizer to grow, but fertilizer can speed up their growth.
Seeds of Crops can be fertilized with Pigsy Droppings to grow into Seedlings immediately. This fertilizer can only be obtained from tamed yellow Pigsies, Night Pigsies or Night Hoglets (Pets) after they've been fed their exact favorite type of Food.
Alternatively, Seedlings can be fertilized to grow into Crops faster (for example, fertilized Wheat Seedlings will grow into Weed Sprouts in ca. 2:30 minutes, while unfertilized Wheat Seedlings will take 11:30 minutes for this step), but not after the Seeds themselves have already been fertilized. Which means you can fertilize each Crop only once. Sprouts cannot be fertilized anymore at all in any case.
Fallow Seeds cannot be fertilized either.
Unfertilized Wheat needs approx. 50 minutes (real-life-time) to grow from a Seed into a ripe plant (even if you're offline). Lettuce needs about 60 minutes, Turnips not much longer (ca. 61 minutes), Horned Melons need ca. 90 minutes until they're ripe.
You can place Seeds into the slots of Wood Planters (there's a bug that might render the Seeds themselves invisible, but their sparkles might show instead), Flower Pots, Placemats, Galactic Grav-Chambers, Hidden Temple Altars and on Wall Shelves, but they won't grow into Crops there, they will just be conserved to be displayed for decoration.
If you remove the tilled land from under a Seed, the Seed will turn fallow, but still keep on floating in mid-air (in comparison to non-solid blocks like Red Mushrooms or Tallgrass). If you remove the tilled land from under a Seedling or Sprout, they will revert back to Seeds, and will be fallow.